I found 1280xwhatever to be quite nice. It is supported in various resolutions. For me, 1280x720, my wife's screen, 1280x1024. In one of my programs I took the desktop resolution, figured out the ratio then multiplied 1280 by that ratio to fit it to the screen (or just use the desktop resolution).

On my current game it is in 800x600, which is simple to use in windowed mode and for full screen I set it to a fullscreen window the size of a desktop resolution, and then stretch everything to fit using ALLEGRO_TRANSFORM.